WFP052797 Plants Know the Way to Grow

As students study plants they are sure to notice of the cells, so calcium and auxin accumulate at that plants seem to “know” which way to grow. the lower side of the root cap. Germinating seedlings emerge above the soil and If root and shoot cells were affected in the same grow upward. On a windowsill plants turn way by differential auxin concentration, plants toward light. Observations that downward would grow in circles. Fortunately, auxin stimu- growth of root (toward water) and upward growth lates cell elongation in shoots but inhibits it in of the shoot (toward light) are critical to a plant’s roots. The same system allows roots to bend •survival lead to questions such as: toward the source of gravity (positive • Why do plants grow up? gravitropism) and shoots to bend away from it • Why do plants grow toward light? (negative gravitropism). Which response is stronger - growing up or • An important control is lacking from experiments growing toward light? done so far - we cannot eliminate gravity while on Is the response to light related to a particular Earth. Only experiments in space demonstrate color of light? plant response in the absence of gravity. Future The Fast Ups Plants and Downs experiments of Life in this document FastThe PlantsLight Sideexperiments performed in conjunc- address these questions which have intrigued tion with NASA on Space Shuttle missions may plant scientists for years. Though we aren’t sure answer some of the questions about tropisms in what causes these responses, the following microgravity (see Wisconsin Fast Plants Notes, information explores current hypotheses. Spring 1995 issue, Vol. 8, No. 1).

How do plants know which way is up (and which Negative gravitropism leads plants to grow up out is down)? The response (gravitropism) seems to of the Earth but growth of the shoot directly be mediated by the root cap and shoot tip. In toward the sun is even more advantageous to a roots, perception of the direction of gravity photosynthetic organism. This response (photot- appears to depend on the settling of dense ropism) is apparently mediated by the shoot tip particles (organelles called amyloplasts) in spe- and has been mostly studied in coleoptiles (the cialized root cap cells. When the plant is turned, sheath around cereal grain shoots). Unequal within minutes amyloplasts sink toward the auxin distribution also seems to be involved, with source of gravity, to the side of the cell that is auxin apparently transported away from the currently "down." lighted side toward the darker side of the shoot. Curving of the roots results from asym- Since auxin stimulates cell elongation in shoots, metric growth as the root elongates. this produces unequal growth on the two sides of Picture two the shoot and the shoot bends toward the light. layers of cells Research has shown phototropism to be a re- attached to sponse to blue light but the receptor is not well each other. If established. The receptor is not a phytochrome, only one layer elongates, it forces the two layers and experiments indicate that a likely candidate to bend. Movement will be away from the side is a flavoprotein. where elongation occurs. Phototropic and gravitropic responses share Elongation of plant cells is affected by a plant several properties. A stimulus (light or gravity) growth hormone called auxin. Auxin concentra- leads to unequal distribution of auxin. The tion appears to parallel amyloplast distribution change in auxin distribution almost certainly leading to a gravity induced auxin gradient results from a lateral migration of auxin rather across the shoot or root tip. In root studies, than from differential synthesis or degradation. calcium levels rise where amyloplasts accumu- Root or shoot bending is due to differential cell late. This may activate pumps in the membrane enlargement in response to differing auxin to pump calcium and auxin out of the lower side concentrations. © 1995 Wisconsin Fast Plants, University of Wisconsin, College of Agricultural and Life Sciences Department of Plant Pathology, 1630 Linden Drive, Madison, WI 53706 1-800-462-7417 [email protected] There is a lot more room for research! Experi- 2. Observe 1 and 2 days later and record what ments in this document introduce the phenom- Gravitropismyou see. Replace - Experiment cap tightly 2 and return the enaMaterials of gravi- and phototropism. Your experimen- chamber to the same horizontal position. tation is limited only by your imagination - try 3. When you are done observing, seedlings may combinations of growth in dark or light, in differ- be planted in potting mix and used for fur- ent colors of light, in different growth orienta- ther study. tions, and so on.

• Fast Plants seeds (phototropism, gravitropism 1. Set up another windowless chamber. experiment 1) or 3- or 4-day-old Fast Plants 2. Add just enough water to cover the bottom of • seedlings (gravitropism experiment 2) the film can. • 35 mm opaque film cans • 3. Wet a paper towel wick strip. Stick it to a mylar plastic sheets, assorted colors plastic grid strip (see Hints for directions for Make• a phototropism chamber single hole paper punch making grid strips). • clear plastic tape (3/4" width) • 4. Put the wick and grid strip on the inside wall black plastic electrical tape Figure 2 • paper toweling or blotting paper of the film can, with the grid strip between scissors the wick and the side of the can. The wick should be in contact with the water in the film can. 1. Punch three holes in the film can, evenly 5. Stick a seedling to spaced around the can about 1 cm down from the wick strip, the rim. with the cotyle- 2. Tape a square of colored plastic over each dons against the hole with clear tape. Cover all holes to main- wick and the tain humidity inside the chamber. hypocotyl pointing out into the can (see Figure 3 3. Place a small amount of water in the bottom Figure 2). of the can. 6. Push the wick and grid strip 4. Place three paper towel wick strips on the below the lip of the can and care- inside of the can, one between each pair of fully colored windows. Allow the water to wick up replace the toweling via capillary action. the lid 5. Place 3 seeds on each wick strip in a triangle, (see slightly below window level. Figure Gravitropism - Experiment 1 3). 6. Carefully replace the lid on the can and place in under a light bank. 7. Make a hypocotyl hypothesis: what do you 7. After 72 hours, open the chamber and ob- think will happen to your serve which way your plants have grown. seedling? After 4 to 6 hours, peek!

1.up Sow 3 seeds on a square of wet blotting paper 8. What additional experiments can you design based on your observations? in marka windowless top chamber (see Figure 1). Placeof chamber the chamber horizontallyFigure 1 and mark which side is up with a pen.

3 Fast Plants seeds

wet blotting paper Hints for making and using tropism chambers

• Cover all holes in your chambers with plastic tape to maintain the humidity. • Making grid strips:

- Photocopy the section of grid paper below onto a transparency sheet, or - Photocopy a sheet of graph paper (10 mm to the cm) onto a transparency sheet. - Cut strips 4 cm long by 0.5 cm wide. 4 cm

0.5 cm

Extensions

• Construct a phototropism chamber with different sized holes covered with clear plastic to explore the etiolation and the effects of light intensity and angle of incidence. • Construct a set of chambers, with each chamber having a different color plastic over its windows to explore the effect of light color on germination and early growth stages. • Construct a phototropism chamber and cover one or more holes with black electrical tape for all or a portion of your experiment time. • Place a phototropism chamber on its side with the window facing down but allowing some light to enter. Will the response to light or gravity be stronger? • Place your seedling gravitropism cans on a spinning turntable. What will be the re- sponse of the hypocotyl? What will be the effect of varying the distance from center?